Nowadays, automotive parts such as gears, transmissions, and seat recliners are often manufactured by forming a hot-rolled steel sheet, which is carbon steel material for machine structural use prescribed in JIS G 4051, into desired shapes by using a cold forming method and by performing a quenching treatment on the formed steel sheet in order to achieve desired hardness. Therefore, a hot-rolled steel sheet, which is a raw material for parts, is required to have excellent cold formability and hardenability, and various steel sheets have been proposed to date.
For example, Patent Literature 1 discloses a method for manufacturing a softened medium- or high-carbon steel sheet, the method including cold-rolling a hypoeutectoid hot-rolled steel sheet having a chemical composition containing, by mass %, C: 0.1% to 0.8%, Si: 0.15% to 0.40% and Mn: 0.3% to 1.0%, limiting P: 0.03% or less, S: 0.01% or less and T.Al: 0.1% or less, and the balance being Fe and incidental impurities with a soft reduction of 20% or more and 30% or less, sequentially performing three-step annealing including first heating in which the cold-rolled steel sheet is held at a temperature equal to or higher than the Ac1 transformation temperature −50° C. and lower than the Ac1 transformation temperature for 0.5 hours or more (exclusive of a soaking time of 6 hours or more), second heating in which the heated steel sheet is held at a temperature equal to or higher than the Ac1 transformation temperature and equal to or lower than Ac1 transformation temperature +100° C. for 0.5 to 20 hours, and third heating in which the heated steel sheet is held at a temperature equal to or higher than the Ar1 transformation temperature −50° C. and equal to or lower than the Ar1 transformation temperature for 2 to 20 hours, in which the cooling rate from the holding temperature of the second heating to the holding temperature of the third heating is 5° C./h to 30° C./h. The object of the invention according to Patent Literature 1 is to soften a medium- or high-carbon hot-rolled steel sheet so that the steel sheet can be satisfactorily subjected to integral forming of a high degree of working while maintaining hardenability.
In addition, Patent Literature 2 discloses a method for manufacturing a medium- or high-carbon steel sheet excellent in terms of local ductility, the method including annealing a hot-rolled steel sheet containing C: 0.10 to 0.60 mass by using heating at a temperature equal to or higher than the Ac1 transformation temperature, in which a metallographic structure (microstructure) having an amount of α/γ boundaries per unit area of γ of 0.5 μm/μm2 or more is formed at the end of heating at a temperature equal to or higher than the Ac1 transformation temperature, or in which a metallographic structure having a number of undissolved carbides of one or more per 100 μm2 and an amount of α/γ boundaries per unit area of γ of 0.3 μm/μm2 or more is formed at the end of heating at a temperature equal to or higher than the Ac1 transformation temperature, and thereafter cooling the heated steel sheet to a temperature equal to or lower than the Ar1 transformation temperature at a cooling rate of 50° C./h or less. The object of the invention according to Patent Literature 2 is to provide a method for manufacturing a medium- or high-carbon steel sheet as a material with which there is a stable increase in stretch flangeability and with which sufficient hardenability is achieved even after being formed into a part by using a common medium- or high-carbon type steel sheet without adding any special chemical element. In addition, in Patent Literature 2, it is said that a chemical element which improves properties such as hardenability may be added and that, in particular, a minute amount of B added significantly increases hardenability of steel material.
In addition, there is a case where a hot-rolled steel sheet which is used as a raw material to be subjected to press forming is required to have an in-plane anisotropy (Δr) of an r value (Lankford value) of almost 0, that is, a small absolute value for Δr in order to achieve satisfactory roundness or in order to prevent a variation in thickness.